Population is easy. Here’s the list: List of continents by population – Wikipedia
Asia is number one with almost 60% of the world’s population. As we also have to add Europe to this, we’re nearly at 70%. That’s huge! Africa is only 15% and North America less than 8%. This means that by numbers alone, the Eurasian continent is extremely powerful.
Next would be the use of military technologies. And Europe, China, Russia, Japan, the USA, Canada, India and Australia all have very modern technological advances. The USA focuses mostly on air power and thus has a lot of aircraft carriers while China and Russia focused more on land power so they have mostly tanks. Russia and China also have anti-satellite missiles and China has proven that they can destroy any satellite in space if they want to. Once they do that, the US forces would be extremely weakened as they depend a lot on communication and espionage satellites and on the GPS system. (Desert Storm was won by the allied forces simply because they used GPS to navigate through the desert.)
Africa and South America have more technological challenges as these continents are dealing with a lot of poverty and not enough technological investments. Africa is also torn apart by various wars and conflicts so they are struggling on technological levels.
Next, firepower. Countries with nuclear weapons happen to be in North America and Asia. Australia doesn’t want them but has the knowledge and materials to build them, if need be. Africa and South America lach the knowledge. So we would have only two important Nuclear superpowers. France, Britain, Israel, Pakistan, India, China and Russia all have nukes and they would definitely outnumber all the nukes North America has so in firepower, Eurasia wins.
Now, without the nukes, Eurasia would still be stronger than the USA as they would start by taking down all satellites in orbit. That would basically put an end to most of the American communication lines. Russia and China depend a lot less on communications by satellite. And while the American aircraft carriers add a lot of firepower, they are also just a few targets for hostile troops to attack and sink to disable the aerial powers of the USA over sea.
In combat ships and submarines, the Eurasian continent would dominate and could consider an invasion of the North American continent. The best way would be going through Russia to Alaska, either by sea or over the ice of the North Pole. As Russian troops have specialized in operating in arctic areas like Siberia, they could manage to set up landing bases in Alaska and Canada before moving further southwards.
But as Eurasian would join forces, they would also have several aircraft carriers themselves that can be useful for such an invasion. As they have taken down all satellites, the North americans would have troubles discovering where those enemy ships would be. From which side would they start their approach and attack?
Of course, the USA could also launch their own attacks but while they have air superiority, they lack the power of ground forces. The number of tanks owned by Russia, China, India and the other countries is huge. And the Eurasian troops just have to keep their anti-aircraft weapons pointed at the sky to destroy any incoming planes. And as Eurasia can shoot down satellites, there’s just no height at which hostile planes would be safe. They would need stealth technology to make attacks and once they’ve made their attacks, those stealth planes still need to return home as they have now been detected.
Australia, Africa and South America are no match against all this to begin with, anyways.
But what if we consider Europe to be a continent? It would not be fair as Russia would then have to be split in two parts. Most Russian troops are located in the European part, so dividing Russian troops over Asia and Europe wouldn’t be fair either. The division of Europe and Asia is more politically and historical and it’s not even exactly clear where you would have to do the division, as this line has changed throughout history. Historically, the Black Sea and Sea of Marmara made the division line but nowadays it’s the Ural and Caucasus mountain ranges.
To make it even more complex, as Europe and Asia are connected to Africa, they also sometimes mention the Afro-Eurasia continent! This would have over 85% of the world population… It would also be half of all the land on this planet.
But back to the division of Europe and Asia. They would still be very powerful and Asia would still be capable of shooting down satellites. Asia still has the biggest population and would still be technologically advanced, just like Europe. China, Japan and South Korea would be their most important technical centers while in Europe, Russia, Britain, France, Germany and the Netherlands would also be important technological centers. They would also have nuclear capabilities and military power is still string for both.
As the USA still depends a lot on the use of satellites, both Europe and Asia would still be reasonably stronger as they’ve focused more on naval and land forces. An invasion of America would be more difficult, though. They can’t attack North America from both sides. (They could when they were just one nation.) As Europe and Asia could be hostile towards one another, they can’t consider an attack on North America without exposing one of their flanks, as they can both easily invade one another over land. Both have plenty of tanks and artillery for such maneuvers. So this would be more a stalemate situation as any aggressor would likely be attacked by multiple others.
If Eurasia is just a single nation then they would have enough force to take control over the whole world. Australia with a population of 40 million would easily be overrun. Africa has a much larger population but lacks good quality weapons. Same is true for South America although they are further away and thus need a lot of naval power to set up a landing base for invasions. North America would be the biggest challenge as it has air superiority, requiring a lot of anti-aircraft weapons. Those North American forces would do the most damage to the Eurasian troops but still would not be able to stop any invasion that starts in Canada, Alaska and Greenland! Especially Greenland is a weak spot as the population in Greenland lives mostly near the coats. Inlands, it’s mostly bare so ideal for troops to pass through. (Very hostile territory too!)